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Solid Fluidized Bed with Thermally Induced Inter-Particle Forces Jaber Shabanian Ecole Polytechnique De Montreal, Canada

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Solid Fluidized Bed with Thermally Induced Inter-Particle Forces Jaber Shabanian Ecole Polytechnique De Montreal, Canada Engineering Conferences International ECI Digital Archives 10th International Conference on Circulating Fluidized Beds and Fluidization Technology - Refereed Proceedings CFB-10 Spring 5-3-2011 Fluidization Behavior in a Gas- Solid Fluidized Bed with Thermally Induced Inter-Particle Forces Jaber Shabanian Ecole Polytechnique de Montreal, Canada Farzam Fotovat École Polytechnique, Canada Jamal Chaouki Ecole Polytechnique Montreal, [email protected] Jonathan Bouffard Ecole Polytechnique Montreal Follow this and additional works at: http://dc.engconfintl.org/cfb10 Part of the Chemical Engineering Commons Recommended Citation Jaber Shabanian, Farzam Fotovat, Jamal Chaouki, and Jonathan Bouffard, "Fluidization Behavior in a Gas- Solid Fluidized Bed with Thermally Induced Inter-Particle Forces" in "10th International Conference on Circulating Fluidized Beds and Fluidization Technology - CFB-10", T. Knowlton, PSRI Eds, ECI Symposium Series, (2013). http://dc.engconfintl.org/cfb10/45 This Conference Proceeding is brought to you for free and open access by the Refereed Proceedings at ECI Digital Archives. It has been accepted for inclusion in 10th International Conference on Circulating Fluidized Beds and Fluidization Technology - CFB-10 by an authorized administrator of ECI Digital Archives. For more information, please contact [email protected]. FLUIDIZATION BEHAVIOR IN A GAS-SOLID FLUIDIZED BED WITH THERMALLY INDUCED INTER-PARTICLE FORCES Jaber Shabanian, Farzam Fotovat, Jonathan Bouffard, Jamal Chaouki Department of Chemical Engineering, Ecole Polytechnique de Montreal, Montreal, Quebec, Canada Corresponding author: Tel.: +1-514-340-4711 X 4034; fax: +1-514-340-4159. E-mail address: [email protected]. ABSTRACT In this work, a new approach for increasing and controlling inter-particle forces (IPFs) was applied. This method used a spherical inert particle coated with a polymer material having a low glass transition temperature. Since IPFs depend on the temperature of the coated particles, they can be easily controlled by the temperature of the inlet air. For this reason, the temperature of the system was varied uniformly near the glass transition temperature of the polymer, between 20 – 40oC, to investigate the effect of IPFs on fluidization behavior at low and high gas velocities. INTRODUCTION Particle size, shape, roughness and density as well as inter-particle forces (IPFs) are among the most important parameters affecting the flow dynamics of powder materials. In regard to the significance of IPFs, for instance, there is no doubt that IPFs dominate the fluidization behavior of Geldart Group C particles, which leads to a completely different behavior compared to the other groups of Geldart’s classification with low or no IPFs. Investigations into the influence of IPFs on the behavior of a gas-solid fluidized bed have been carried out using different methods. Most importantly, however, controlling the level of IPFs to have a uniform cohesion throughout the particulate media is not an easy task. Methods that have been used include the following: increasing the amount of Van der Waals forces by decreasing the mean size of particles (1); increasing the amount of capillary forces by the addition of a cohesive agent into the bed (2); creation of a magnetic field around the bed (3) or raising the bed temperature to a high value (4). Each of these methods has specific difficulties in practice. For the first method it is very difficult to control particle shape and surface roughness. Due to these difficulties, the use of Van der Waals forces to study the effect of IPFs on fluidization characteristics is a complex procedure. By increasing the size of particles, the hydrodynamic forces become dominant compared to the IPFs. For larger particles, an increase in IPFs must be induced by the addition of a cohesive agent into the particulate system. One of the most popular methods to conduct this technique is by the presence of wet capillary bonds created by an interstitial liquid between the particles. The problem with this approach is that it is challenging to have a uniform distribution of the agent throughout the whole bed, which leads to force anisotropy 1 inside the particulate system (5). The other problem with this approach is that it restricts the fluidization study at low superficial gas velocities. To employ the third method an expensive set-up is needed to generate a magnetic field with the help of a costly electromagnetic coil system. The other problem of this approach is that the ferromagnetic particles attract themselves when they are parallel to the magnetic field and repel each other when they are perpendicular, which, consequently, causes anisotropic attraction/repulsion in the bulk materials (6). For the last method, the first problem is that it is costly to have an apparatus operating at high temperatures. Secondly, the lack of proper measurement techniques at high temperatures is the other difficulty with this strategy (7). In this work, a novel approach is proposed to induce IPFs inside a particulate media. It involves large particles that are not significantly influenced by the Van der Walls forces and other colloidal interactions. This technique uses a copolymer of PMMA/PEA (Poly Methyl MethAcrylate/Poly Ethyl Acrylate) contained in a polymer suspension called Eudragit NE30D. The copolymer, which is characterized by a low glass transition temperature, around 9oC, is coated on inert particles by an atomization process. By changing the ambient temperature to which the coated particles are exposed, the polymer adhesion/friction parameters and Young Modulus are modified in a way that the observed cohesive IPFs between the particles are changed significantly. Accordingly, the cohesion between particles can be adjusted by temperature in a stable and reproducible manner. The advantage of this method is that it does not necessitate the addition of any liquid phase, which has to be uniformly distributed into the particulate system. Besides, it allows the fluidization study to be carried out at both low and high superficial gas velocities. In contrast to the magnetic interactions, the forces are not dominant in only one direction, but are located at each contact between the particles. Thus, there is no force anisotropy present except if the temperature is not uniform in the system. The last priority of this technique is that it can be conducted at low temperatures and IPFs are changed in a completely controlled manner by merely a small increment in the system temperature. METHODOLOGY Particle Coating Process The experimental work necessitates having inert particles as base particles, which can accept the copolymer of PMMA/PEA as the coating. A 450-720μm cut of 3 spherical sugar beads (dp=580μm, =1575 kg/m ), which lie in Geldart Group B particles, were chosen as inert particles in this work. Sugar beads were coated by an atomization process with a polymer suspension in water and dried simultaneously to obtain a uniform coating on the particle surface. It was achieved in a spheronizer, which allowed the introduction of air under the rotating disc located inside the bowl. Table 1 presents operating parameters associated with the coating process of the particles. Heated air, which was passed through an electrical heater before entering the processing chamber, was used for adjusting the temperature to the desired operating setpoint and also for concurrent drying of the particles. The temperature was controlled with the help of a controller coupled with an infrared cell, which measured the surface temperature of the particles. In addition, a thermocouple located under the spheronizer disc allowed for measuring the air entrance temperature. The air flow rate was adjusted to the 2 desired value with the help of a rotameter. The coating solution (Water 0.21 kg; PMMA/PEA 0.086 kg; Nonoxynol100 0.004 kg) was added by atomization onto the particles with a Schlick 970 series two-substance atomizer. The atomizer was fed with the solution by a peristaltic pump with a flow rate approximately equal to 1 g/min and compressed air, which allowed the formation of fine droplets. The atomizer gun was arranged in such a manner that the tip of the nozzle was placed at approximately 4 cm from the torus surface to avoid coating losses on the parts of the equipment. The characteristics of the final product are summarized in Table 2. Table 1. Spheronizer’s Operating Parameters Table 2. Final Particles Coating Characteristics Disc rotational rate (rpm) 230 Materials Quantity Air flow rate (cfm) 25 Spherical sugar beads 3.0 (kg) Air temperature (oC) 30 PMMA/PEA 0.10 (kg) Solution flow rate (g/min) 1 Mass percentage of coating 3.4 % Atomization pressure (bar) 2 Coating layer thickness ~ 5 Experimental Set-up and Procedure for Fluidization Study The experimental set-up used for the fluidization study consisted of a fluidization column, which was constructed with a transparent Plexiglas tube with 0.152 m I.D. and 1.5 m in height. Dried and filtered air was introduced into the bed through a perforated plate as the distributor. It contained holes 1 mm in diameter arranged in a triangular pitch. Air was heated with the help of an electrical heater before entering the fluidizing column. Accordingly, it was used to adjust the temperature of the bed to a desired value. Temperature was controlled by means of a PI controller driven by a thermocouple constantly immersed in the bed. A thermocouple located at the windbox allowed measuring the air entrance temperature. Furthermore, the air flow rate was controlled with a calibrated rotameter, which gave rise to a maximum superficial gas velocity of 0.75 m/s in the bed. In this regard, different superficial gas velocities were used for each system and temperature tested, covering both the fixed bed state and bubbling regime. To investigate the effect of IPFs on the fluidization behavior, two systems were studied, uncoated sugar beads and coated sugar beads. Experiments of uncoated sugar beads were carried out at 20oC while the ones for coated sugar beads were conducted at different operating temperatures, 20oC, 30oC and 40oC.
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